To photodeprotect or not: effect of the oxidation state of the sulfur atom of thiochromone derivatives.

Time-resolved spectroscopic experiments, assisted with DFT calculations, were employed to study the photochemical reaction mechanism of (4-oxo-3-phenyl-4 H -thiochromen-2-yl) methoxycarbonyl-caged ethanol (TC) and (1,1-dioxido-4-oxo-3-phenyl-4 H -thiochromen-2-yl) methoxy carbonyl-caged ethanol (TS-PPG) in different solvents. TC went through an intersystem crossing to form the triplet state with π-π* character in acetonitrile (MeCN) and protic solvents. While the n-π* triplet state was generated for TS-PPG in MeCN, which further underwent Paternò-Büchi reaction to give a biradical intermediate. Then, the C-O bond was cleaved, followed by deprotonation. Besides the similar deprotection route in MeCN, another reaction pathway existed in protic solvents, where the C-O bond heterolysis took place via the singlet excited state. The unambiguous mechanism would not only guide the efficient application of TS-PPG, but also help develop more excellent PPGs based on the thiochromone framework.